(i) Field of the Invention
The present invention relates to a transparent conductive laminate in which a transparent conductive a film mainly comprising tin, indium and oxygen is formed on a transparent substrate, and more specifically, it relates to a transparent conductive laminate using an amorphous film as a transparent conductive film and having excellent moist heat resistance and scuff resistance, and an electroluminescence (EL) light-emitting element using this transparent conductive laminate.
(ii) Description of the Prior Art
In recent years, devices and equipments regarding optical electronics have remarkably progressed and prevailed with the increasing demand of information in society. In such circumstance, transparent conductive laminates have widely been used as electrodes of I/O devices such as transparent touch panels, electrodes of display devices such as liquid crystal displays, and electroluminescence displays and electrochromic displays. Further, they have been uses as window electrodes of photoelectric conversion elements such as solar batteries and the like, and electromagnetic shielding films of electromagnetic wave shields.
The transparent conductive laminate is usually constituted of a transparent substrate and a transparent conductive layer formed thereon. Examples of the transparent conductive layer include metallic thin films of gold, silver, platinum, palladium and the like, oxide semiconductor thin films of indium oxide, tin (IV) oxide, zinc oxide and the like, and multi-layer thin films comprising a laminate of a metallic oxide and a metal. The metallic thin films are excellent in conductivity but poor in transparency. On the contrary, the oxide semiconductor thin films are slightly poor in conductivity in general but excellent in transparency. Of these oxide semiconductor thin films, the thin films comprising indium, tin and oxygen, which are also called ITO (indium tin oxide) films, are excellent in conductivity and transparency, and in addition, and can easily be formed into electrode patterns by etching. For these features, the ITO films have widely been utilized. The resistivity and the light transmittance of the ITO films are usually in the range of about 5xc3x9710xe2x88x925 to 1xc3x9710xe2x88x923 xcexa9xc2x7cm and in the range of 80 to 90%, respectively.
As factors for the performance evaluation of the transparent conductive laminate, there are chemical stability such as moist heat resistance and physical strength such as scuff resistance in addition to the electric resistance and the light transmittance. With regard to the ITO film formed at a low temperature, its electric resistance usually changes depending on the amount of oxygen in the film, so that the electric resistance noticeably changes by a heat treatment or a moist heat treatment. Accordingly, the thus formed ITO film has the problem of chemical stability. The transparent conductive laminate having the thus formed ITO film is finally used as a transparent electrode of a product such as a liquid crystal display or a transparent touch panel, but in this case, if the performance of the transparent conductive laminate changes, a trouble might occur in the product. Moreover, the ITO film formed at a low temperature is liable to be scuffed, and when the ITO film is used in contact with other members as in the transparent touch panel, mechanical strength such as scuff resistance is required to be improved. Furthermore, such an ITO film is chemically unstable, and when the ITO film is coated with another organic substance as in an electroluminescence light-emitting element, the quality of the ITO film itself changes with time. Thus, it is necessary to obtain the chemically stable ITO film.
As means for solving the above-mentioned problem, there usually are a method which comprises heating a substrate at the time of the formation of the ITO film to obtain the crystalline ITO film, and another method which comprises subjecting the ITO film formed at room temperature to a heat treatment to obtain the crystalline ITO film [e.g., Japanese Patent Publication 15536/1991 (JP, B2, 3-15536), and Japanese Patent Application Laid-open Nos. 100260/1989 (JP, A, 1-100260), 194943/1990 (JP, A, 2-194943) and 276630/1990 (JP, A, 2-276630)]. Both of these methods take the means for obtaining the crystallized ITO film by the heat treatment. In the methods, it is utilized that the crystallization of the ITO film permits the formation of the film stable to heat and moisture and hence the improvement of the moist heat resistance and the scuff resistance.
A temperature at which the ITO film is crystallized depends upon the method and the conditions of the film formation, but it is usually 180xc2x0 C. or more.
The crystalline ITO film formed by the heating film formation or the heat treatment after the film formation usually comprises crystallites (or grains) having a diameter of from several xcexcm to several tens xcexcm. If the size of the crystallites is small, a large number of boundaries between the crystallites are in the film, and therefore a gas in the atmosphere easily permeates through the boundaries, so that the moist heat resistance deteriorates. In order to prevent this permeation, the size of the crystallites is required to be enlarged, and for this enlargement, it is necessary to increase the temperature of the film formation or the temperature of the heat treatment after the film formation. For sake of the improvement of moist heat resistance, it is effective that the film formation or the heat treatment after the film formation is carried out at a temperature of about 400xc2x0 C.
One of the products which requires transparent electrodes is an electroluminescence light-emitting element. The known electroluminescence light-emitting element can be manufactured by forming a light-emitting layer and a back surface electrode in turn on a transparent conductive laminate in which the transparent conductive layer is formed on the transparent substrate. For the purpose of effectively applying an electric field to the light-emitting layer to improve a light-emitting luminance, a dielectric layer having a high dielectric constant is usually inserted between the light-emitting layer and the back surface electrode. Further, in order to prevent the light-emitting layer from deteriorating due to water vapor contained in the atmosphere, all or a part of the light-emitting surface of the electroluminescence light-emitting element is usually covered with a moisture barrier film. In this case, usually, the transparent conductive layer is made of the ITO film or the like, and the light-emitting layer is made of zinc sulfide, cadmium sulfide or zinc selenide, and the back surface electrode is made of aluminum or carbon.
Since the electroluminescence light-emitting element can be obtained in the form of a thin sheet, there is expected its application to a use in which such a shape is required, for example, a back light of a liquid crystal display or an emitting element of the dial of a watch.
The electroluminescence light-emitting element is characterized by being obtained in the form of the thin sheet, but its light-emitting durability is poorer as compared with a fluorescent tube which is a conventional light source. For this reason, the electroluminescence light-emitting element has not actually been prevailed so far. Thus, it has been desired to develop the electroluminescence light-emitting element by which the above-mentioned problem can be solved. In particular, the electroluminescence light-emitting element in which a polymeric film is used as the transparent substrate can be applied in a wide utilization range, because it can emit the light while curved.
As one factor by which the luminance of the electroluminescence light-emitting element deteriorates during the continuous light emission, there is the deterioration of the ITO film of the transparent conductive layer used as the transparent electrode as described above. The transparent conductive layer for the transparent electrode of the electroluminescence light-emitting element is required to have a visible light transmittance of 80% or more and a surface resistance of 1000 xcexa9/xe2x96xa1 or less. In addition, since the transparent electrode is used in contact with the light-emitting layer, it must be stable to a material for the light-emitting layer.
As described hereinbefore, the characteristics of the transparent conductive laminate having the formed crystalline ITO film depend upon the size of the crystallites of the ITO film, and therefore the transparent conductive laminate having the excellent moist heat resistance and scuff resistance cannot always be obtained. In order to form the transparent conductive laminate which is excellent in the moist heat resistance and the scuff resistance, the temperature of the film formation or the temperature of the heat treatment after the film formation is strictly controlled to regulate the size of the crystallite. If the temperature of the film formation or the temperature of the heat treatment is 400xc2x0 C. or more, the transparent conductive laminate having the excellent moist heat resistance and scuff resistance can relatively easily be obtained, but when the transparent conductive laminate is formed by the use of a transparent molded article of a polymer having flexibility, the molded article of the polymer cannot be heated up to 400xc2x0 C., because a heat-resistant temperature of the molded article of the polymer is usually in the range of about 120 to 250xc2x0 C.
In the case that a glass substrate is used as the transparent substrate, a crystalline ITO film having a low electric resistance value can be formed as the transparent conductive layer by either of a manner of forming the ITO film at a film formation temperature of 400xc2x0 C. or more and a manner of forming the film at a low temperature and then carrying out the heat treatment at 400xc2x0 C. or more. In the case that the molded article of the polymer is used as the transparent substrate, however, the upper limit of the temperature of the film formation or the temperature of the heat treatment after the film formation is limited to the heat-resistant temperature of the molded article of the polymer. The upper limit temperature is usually 250xc2x0 C. or less. The ITO film formed at a low temperature, particularly at room temperature has many structural faults and is chemically unstable.
In the electroluminescence light-emitting element in which the ITO film formed at the low temperature is used as the transparent electrode, a reaction of the material of the light-emitting layer with the ITO film in the vicinity of the interface between the light-emitting layer and the ITO film is accelerated during the light emission by an applied electric field, so that the quality of the ITO film changes and the light-emitting luminance deteriorates, with the result that a practically sufficient durability cannot be obtained. In order to solve this problem, the ITO film in which the film quality does not change by the contact with the light-emitting layer and the electric field applied for the light emission and which is excellent in the chemical stability needs to be used as the transparent conductive layer.
In practice, in the electroluminescence light-emitting element, it is required that when the light emission is continued under conditions of 40xc2x0 C. and a relative humidity of 90%, a light emission durability time of a light-emitting luminance/initial light-emitting luminance change ratio I/I0=0.5 is 200 hours or more. Needless to say, the higher the light-emitting luminance is, the more desirable it is.
In view of the above-mentioned circumstances, the present invention has been intended, and an object of the present invention is to provide a transparent conductive laminate in which an amorphous ITO film having improved moist heat resistance and scuff resistance is formed on a main surface of a transparent substrate. A conventional amorphous ITO film is unstable to environment, and when the amorphous ITO film is merely exposed to the atmosphere, the electric resistance of the conventional amorphous ITO film rises due to water vapor in the atmosphere. In addition, the mechanical strength of the amorphous ITO film is so weak that it is scuffed by slight friction. Hence, the conventional amorphous ITO film is inferior to a crystalline ITO film in moist heat resistance and scuff resistance. On the contrary, according to the present invention, a good amorphous ITO film having an excellent stability and mechanical strength can be obtained, and by the use of this amorphous ITO film, a transparent conductive laminate which is excellent in the moist heat resistance and the scuff resistance can be supplied. When this laminate is used as the transparent electrode of an electroluminescence light-emitting element, a particularly remarkable effect can be exerted, and since the chemical instability of the ITO film which causes the deterioration of luminance during continuous light emission can be eliminated, the electroluminescence light-emitting element in which the durability of the continuous light emission is improved can be provided.
The present inventors have intensively researched to solve the above-mentioned problem, and as a result, it has been found that, in a transparent conductive laminate in which an amorphous transparent conductive layer mainly comprising indium, tin and oxygen is formed on a transparent substrate, the transparent conductive layer which holds an amorphous state even after subjected to the heat treatment is chemically and physically stable and excellent in the moist heat resistance and the scuff resistance. This transparent conductive layer can be prepared by depositing an amorphous material mainly comprising the oxides of indium and tin and having a resistivity of 1xc3x9710xe2x88x922 xcexa9xc2x7cm or more by a sputtering process, and then subjecting the material to a heat treatment to form the amorphous transparent conductive layer having a resistivity of 1xc3x9710xe2x88x922 xcexa9xc2x7cm or less. Thus, the present inventors have found that the transparent conductive laminate having a sufficiently low electric resistance value can be obtained by this treatment, and on the basis of this finding, the present invention has been completed. The electron mobility of this transparent conductive laminate is 20 cm2/(Vxc2x7sec) or more, and even when the transparent conductive laminate is subjected to the heat treatment, its value is maintained at 20 cm2/(Vxc2x7sec) or more and an electron concentration increases. Furthermore, when this transparent conductive laminate is used as the transparent electrode of the electroluminescence light-emitting element, the deterioration of the light-emitting luminance by the continuous light emission can be inhibited to such a remarkable degree as not to be seen in a conventional case.
The method for forming the ITO film having the high resistivity by the sputtering process under a high oxygen concentration atmosphere has been disclosed in Japanese Patent Application Laid-open No. 36703/1991 (JP, A, 3-36703), and there is herein described an ITO film having a surface resistance value in the range of 1 Mxcexa9/xe2x96xa1 to several Gxcexa9/xe2x96xa1 which can be manufactured by sputtering or vapor deposition in the atmosphere of a heightened oxygen partial pressure. However, the ITO film having such a high electric resistance value, needless to say, cannot directly be used as the transparent electrode of the electroluminescence light-emitting element.
Furthermore, in Japanese Patent Application Laid-open No. 145325/1989 (JP, A, 1-143525), there has been disclosed a method for preparing a transparent conductive film having improved mechanical durability by forming the ITO film under a high oxygen concentration atmosphere by the sputtering process, and then subjecting the film to the heat treatment. In this publication, the amount of an oxygen gas to be introduced is regulated so that a surface resistance change ratio R/R0 (R0=a surface resistance before the heating, and R=a surface resistance after the heating) of the transparent conductive film subjected to the heating at a temperature of 150xc2x0 C. for 30 minutes after the film formation may be 0.8xe2x89xa6R/R0xe2x89xa61.0, preferably may be substantially 1, whereby the transparent conductive film having a high keystroke resistance can be obtained. However, in order to obtain the transparent conductive film having the sufficient light-emitting durability as the transparent electrode of the electroluminescence light-emitting element, this preparation method is insufficient as described in the undermentioned comparative example. In the present invention, the ITO film of 1xc3x9710xe2x88x922 xcexa9xc2x7cm or more is first formed by the sputtering process under the high oxygen concentration atmosphere, but in the case of a film thickness of 100 nm, this value corresponds to 1000 xcexa9/xe2x96xa1 or more. That is to say, in the present invention, if the resistivity can be lowered by the heat treatment, it is preferred that the ITO film having the highest possible resistivity is first formed. By the utilization of the ITO film whose resistivity before the heat treatment is 1xc3x9710xe2x88x922 xcexa9xc2x7cm or less, i.e., 1000 xcexa9/xe2x96xa1 or less in the case of a film thickness of 100 nm, a sufficient effect cannot be obtained, when the ITO film is used as the transparent electrode of the electroluminescence light-emitting element.
One aspect of the present invention is directed to a transparent conductive laminate in which an amorphous transparent conductive layer (B) mainly comprising indium, tin and oxygen is formed on one main surface of a transparent substrate (A), said transparent conductive layer maintaining an amorphous state after subjected to a heat treatment.
Another aspect of the present invention is directed to a transparent conductive laminate in which an amorphous transparent conductive layer (B) mainly comprising indium, tin and oxygen and having a resistivity of 1xc3x9710xe2x88x922 xcexa9xc2x7cm or more is formed on one main surface of a transparent substrate (A), the resistivity of said transparent conductive laminate becoming 1xc3x9710xe2x88x922 xcexa9xc2x7cm or less by a heat treatment while the amorphous state of said transparent conductive layer is maintained, and another transparent conductive laminate whose resistivity is decreased to 1xc3x9710xe2x88x922 xcexa9xc2x7cm or less by a heat treatment.
Still another aspect of the present invention is directed to a transparent conductive laminate in which an amorphous transparent conductive layer (B) mainly comprising indium, tin and oxygen and having an electron mobility of 20 cm2/(Vxc2x7sec) or more is formed on one main surface of a transparent substrate (A), said transparent conductive layer maintaining an electron mobility of 20 cm2/(Vxc2x7sec) or more and an amorphous state by a heat treatment, a transparent conductive laminate in which the electron density of the transparent conductive layer (B) is increased by the heat treatment, and a transparent conductive laminate in which the electron density is increased, while an electron mobility of 20 cm2/(Vxc2x7sec) or more and the amorphous state are maintained.
The transparent conductive layer (B) is preferably formed by a sputtering process under a high oxygen concentration atmosphere, and the transparent substrate (A) is preferably a molded article of a transparent polymer.
The heat treatment is preferably carried out in the range of 80 to 180xc2x0 C. in the air, in an atmosphere of an inert gas such as nitrogen or in vacuo. Moreover, between the transparent substrate (A) and the transparent conductive layer (B), a metal thin layer may be formed.
Furthermore, the present invention is directed to an electroluminescence light-emitting element in which a light-emitting layer (C) containing at least zinc sulfide and a back surface electrode (D) are formed in turn on the conductive surface of a transparent conductive laminate, the above-mentioned transparent conductive laminate being used in said electroluminescence light-emitting element. This element can exert a noticeable effect, when driven by a power source superposing a DC component to an AC component.